1. Field of the Invention
The present invention relates generally to methods of providing a releasable attachment between a substrate and a base and, more particularly, pertains to a releasable attachment which provides a heat transfer path and, optionally, an electrical conduction path between the substrate and the base.
2. Description of the Prior Art
Microelectronic subsystems and some microelectronic modules are too expensive to be thrown away if only a small part of the circuitry therein fails. Accordingly, it is desirable to be able to remove a defective module from a larger network in which it is interconnected without causing additional damage to the subsystem. Relatively rigid mechanical fasteners such as screws and spring-loaded clamps, for example, have been used to provide an easily releaseable attachment between a substrate on which microelectronic circuitry is disposed and a base or chassis which provides support for the substrate.
However, an attachment between a substrate and a base usually is relied upon to provide more than mechanical support. Heat transfer from the circuitry on the substrate to the base is almost always an important consideration. The mechanical fasteners mentioned are frequently formed of a thermally insulating material such as nylon. These contribute little to heat transfer. The interface between the botton surface of the substrate and the opposing surface of the base is relied upon to conduct away from the substrate that portion of the heat which is not dissipated by convection.
Where convection is not an available contributor to heat dissipation, as in the case where the microelectronic subassembly is to be operated in a vacuum or in space, the quality of the interface between the substate and the base becomes an especially important consideration. Where pressure alone holds the substrate to the base, the interface is a relatively poor one for heat transfer. The surfaces are ordinarily quite irregular when viewed microscopically. Therefore, actual contact occurs between the two surfaces over a relatively small percentage of the contact area.
The bottom or attachment side of the substrates requiring mounting is sometimes metallized as, for example, in the case of microwave integrated circuits. The bottom metallization serves as a ground plane for stripline waveguides wherein the rf energy is propagated through a dielectric substrate. Satisfactory performance of such microwave integrated circuitry requires very low electrical resistance in the interconnections between the ground planes on the many substrates in a system. The interconnection paths include the bases which may be, for example, aluminum chassis. Relatively high resistance interconnections introduce parasitic impedances into the circuitry. Low resistance, relatively permanent attachments between a substrate and a base have long been made through the use of solder and adhesives such as the epoxy resins which are mixed or filled with metallic powders. Such attachments also serve to conform the surfaces of the substrate and base and, thus, enhance heat transfer also. However, these prior art attachment techniques have the drawback that they are difficult and expensive to release when it is desired to detach a particular module.
3. Prior Art Statement
The following publications are regarded as having pertinence to this invention.
(a) Crane, U.S. Pat. No. 4,012,832, "Method for Non-destructive Removal of Semiconductor Devices," issued Mar. 22, 1977.
(b) MIL-STD-1334B, "Military Standard, Process for Barrier Coating of Anti-Friction Bearings," 4 January 1977.
The patent to Crane discloses a method for releasably attaching semiconductor devices to a substrate and releasably attaching substrates to a base wherein the attachment comprises two layers of conductive adhesive plastic separated by a small chip of conductive alloy which melts above the curing temperature of the adhesive plastic. Release is accomplished by melting the conductive alloy.
The Military Standard discloses that the release agent discussed hereinafter and described as the preferred release agent for practicing this invention is known to have been used for the protection of electrical contacts against lubricant contamination. It is believed that the contacts referred to in this publication are the type used for the movable and stationary contacts of electromagnetic relay devices.